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一次多分叉多接地的空中触发闪电过程
An Altitude triggered Lightning with Multiple Branches and Ground Contacts
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摘要: 该文分析了2007年广州从化人工引雷试验中，6月30日一次空中触发负极性闪电的光学和电学观测资料，结果表明：此次空中触发闪电的上行正先导和下行负先导分别出现了多个分叉。上行正先导早于下行负先导4.93 ms始发，上行正先导初始阶段的二维平均速度为104 m/s量级，后增加到105 m/s量级。下行负先导经历了3次分叉后分为4个分支接地，其中有两个接地分支一直持续发光到闪电放电结束, 观测到下行负先导的二维平均速度为1.69×1 05 m/s。小回击之后上行正先导也出现了多个分支，高速摄像和宽带干涉仪对这些分支的观测结果基本一致。小回击之后，初始长连续电流过程的持续时间为178.6 ms。Abstract: Field experiments on artificially rocket triggered lightning are conducted in summer of 2007 in Conghua, Guangdong Province by Lightning Physics and Protection Engineering Laboratory of Chinese Academy of Meteorological Sciences. The lumin osity characteristics of lighting are captured simultaneously by a Redlake Motion Pro HS 4 high speed camera at 5000 frames per second (fps) 630 m away. Field mill, magnetic loop antenna, broadband interferometers, slow antenna and fast an tenna are installed at the experiment site. The time constants of slow and fast antenna are 6 s and 2 ms respectively, and the bandwidth of broadband interferom eters to locate the lightning is from 25 MHz to 290 MHz. One lightning is succes sfully triggered at 13:58:33 30 June 2007. The top of the wire is 335 m height and the bottom of it is 170 m height at that time. The records of the electric field on the land surface show that it is a negative altitude triggered lightning. No return stroke is observed in luminosity, and the initial continue current p rocess of this flash is 178.6 ms. Bi directional leader process occurs be fore mini return stroke and upward stepped leader branches of this altitude triggered lightning occur after that. The upward positive leader appears 4.93 ms earlier before the downward negative leader. The upward stepped leader propagates at an average speed of the order of 104 m/s in its initial stage, and then becomes 1 05 m/s in magnitude. After the downward negative leader appears, the upward po sitive leader channel becomes brighter than before. The downward negative leader has 3 bifurcation points and produces 4 different ground contacts, and two bran ches connects to ground persisted during the initial continue current process. T he average speed of the downward stepped leader is 1.69×1 05 m/s. After mini r eturn stroke, the upward positive leader above the wire channel propagates along several different branch channels into the cloud, and the average speed of it is 1.05×1 05 m/s. In this experiment, the inducing charge provides the charge of the leader to spread, and the environmental electric field may play a decisive role to the multiple branches and multiple ground contacts of this altitude tr iggered lightning.
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图 1 -10~40 ms的相对电场变化、磁场变化、电晕电流和闪电通道相对积分亮度变化的波形
Fig. 1 The records of fast and slow antenna, corona current, magnetic loop antenna, lumino sity from-10 ms to 40 ms
图 2 -0.8~-0.6 ms相对电场慢变化、磁场变化和电晕电流的波形
Fig. 2 The records of slow antenna, corona current, magnetic loop antenna from-0.8 ms to-0.6 ms
图 3 -0.8~0 ms的高速摄像图片 (为便于观看，提高了-0.8~-0.2 m s图像的对比度)
Fig. 3 Frame sequence of bi-directional leader from-0.8 ms to 0 ms (frame-0.8 mst of rame-0.2 ms has been processed)
图 4 0.2~5.6 ms的部分高速摄像资料
Fig. 4 Frame sequence of bi-directional leader about the minireturn stroke and initial long continue current process
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